Transmission line presence sensor

ABSTRACT

A high frequency generator-receiver system for detecting the presence or proximity of objects including persons or vehicles employs continuous wave transmission of high frequency electromagnetic signals and reception thereof with respect to a transmission line energy coupling system by a receiver circuit cooperating with the transmission line coupling system for detecting changes in the coupled signal energy and for providing corresponding outputs suitable for indication or other reaction to the presence of such proximate objects.

United StatesPatent 119] Ross et a1. Apr. 2-, 1974 [54] TRANSMISSIONLINE PRESENCE SENSOR 3,422,431 1/1969 Hafner 340/258 X 3,270,339 8/1966McEven 340/258 X [75] lnvemors- Gerald Lexmgtm 9* 3,031,643 4/1962Sheftelman.... 340 252 981401111110, Sudbufy; Dav"! 3,068,448 12/1962Mountjoy .1 3401258 c x Lamensdorf, Cambridge, all of I Y Mass- PrimaryExaminerJohn W. Caldwell [73] Assignee: Sperry Rand Corporatiom NewAssistant Partridge York Attorney, Agent, or FirmHoward P. Terry [22]Filed: Dec. 6, 1971 [57] ABSTRACT I PP A high frequencygenerator-receiver system for detec ting the presence or proximity ofobjects including 52 Us C 340 25 R, 340/25 C 340 3 L persons 01'vehicles employs 60111111110118 wave trans- 51 1m. 01. G015 11/00, G08b13/00 mission of high fiequency electmmagnefic Signals and [53] Field ofSearch 340/258 R, 258 C, 38 L, reception thereof with respect to atransmission line 340/38 174/39 energy coupling system by a receivercircuit cooperating with the transmission line coupling system for de-[56] References Cited tecting changes in the coupled signal energy andfor UNITED STATES PATENTS providing corresponding outputs suitable forindication or other reaction to the presence of such proxi- 3,375,5113/1968 Trimble 340/258 mate objects 2,532,231 11/1950 Jarvis 340/38 L3,623,101 11/1971 Grebe 340/258 C X 4 Claims, 11 Drawing Figures SIGNAL8 11 10 GENERATOR T] I 6 e 1 ONE SHOT 0 M, v. I

TO 7 :0 47 FIG. 3

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SIGNAL GENERATOR POWER DIV DER We 0 P81; {/80 SIGNAL PROCESSOR LANE 3SIGNAL PROCESSOR LANE 2 SIGNAL PROCESSOR LANE 1 TRANSMISSION LINEPRESENCE SENSOR BACKGROUND OF THE INVENTION 1. Field of the InventionThe invention relates to radio systems for the detection of the presenceof objects such as persons or vehicles and more particularly relates tothe sensing of the presence of such objects as may move along apassageway or a vehicular traffic lane. Detection is accomplished by anovel transmission line energy coupling system, excited by continuouswave high frequency electromagnetic signals, wherein detectable changesin the level of energy coupling are caused to be sensed by the passageof the object.

2. Description of the Prior Art Prior art devices for sensing thepresence of objects such as persons or vehicles at a particular stationon a vehicular lane or other passageway have not proven adequatelyimmune to disturbances in the environment in which such devices are usedin largest numbers. Furthermore, sensors adequate for sensing vehiclepresence, usually by detecting a magnetic characteristic such as thepresence of or a distortion of a magnetic field, have not been foundfully adequate for detecting the presence of other objects of differentnature such as pedestrians, and vice versa.

Detectors sensing disturbances in unidirectional or low frequency fieldsare relatively insensitive and are particularly susceptible tobackground noise signals, such as electrical noise signals generallypresent in industrial cities, and usually operate on magnetic principlesand therefore generally do not detect the presence of non-magneticobjects, including pedestrians. While relatively simple in nature, theygenerally sense only slowly varying characteristics and therefore oftendo not afford precise, fail-proof operation.

Other sensors operate at kilocycle and higher frequencies; bridge andother such circuit devices are often quite susceptible to the nature oftheir environments and may totally fail in the presence of rain or snow,thereby rendering them unsuited to use where high accuracy, highreliability, and reasonable freedom from generation of false alarms isdemanded. Such equipment often produces undesired false alarms or, toprevent such false alarms, requires'reduction of sensitivity in asignificant degree. Such high frequency sensors are often undesirablysensitive to changing temperature and to drift in supply voltage. Againthe sensitivity of these prior art devices is often dependent on thebalancing of a bridge or the tuning or detuning of resonant circuits andtherefore they also derive their out puts by sensing relatively slowlyvarying parameters rather than rapidly varying parameters.

SUMMARY OF THE INVENTION The present invention relates to high frequencytransmission line coupling means for detecting the presence of objectsthat traverse the coupling device in contacting, substantiallycontacting, or other proximate relation. Such traversal creates anabrupt change in the degree of coupling between substantially parallelcoupled transmission line conductors. The length of the active couplingregion between the parallel transmission lines is adjusted in terms ofthe wave length of the excitation of the transmission line system sothat the normal output of the coupled system is substantially zero.However, the perturbed output is a rapidly changing function of thetransient disturbance rate. The distinctive output of the coupler systemgenerates a useful output pulse for operation of an alarm, a counter, or

other operational or control device.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic plan view of thenovel sensor with details of signal generator and signal processorreceiver circuits with which the sensor is operated.

FIGS. 2 and 3 are block diagrams of apparatus with which the arrangementof FIG. 1 may be used.

FIG. 4 is an elevation view, partly in cross section, of the sensor ofFIG. 1.

FIGS. 5, 6, and 7 are cross section views of alternative constructionsfor the apparatus of FIGS. 1 and 4, taken along the line 5, 6, 7 of FIG.4.

FIG. 8 is an alternative to the arrangement of FIGS. 5, 6, and 7.

FIG. 9 is a graph that is useful in explaining the operation of theinvention.

FIG. 10 is a schematic diagram in plan view of a system incorporatingthe invention for sensing vehicles on a triple-lane roadway.

FIG. 11 is similar to FIG. 10 and shows a plan view of an alternativetriple-lane roadway monitor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The transmission lineobject-presence sensor shown schematically in FIG. 1 detects the actualpresence of an object traversing it in contacting, substantiallycontacting, or other relation by measuring a function of the changecaused by the presence of the object in the amplitude of the electricalsignal coupled between the substantially parallel high frequencytransmission line conductors 1 and 2. The transmission line systemcomprising conductors I and 2 may be of the strip transmission line typeconventionally used in the high frequency art. In particular, thepreferred transmission line system will generally be one capable ofpropagating electromagnetic energy in the low loss, transverseelectromagnetic or TEM mode. i

The high frequency conductors 1 and 2 are affixed to the top surface ofa substrate sheet 3 of low loss dielectric material. 'A metallic groundconducting plane 59 as seen in FIG. 4 is affixed to the bottom of sheet3. At'

one end of the transmission line conductors 1 and 2 are respectivelylocated conventional impedance matching connectors 6 and 7 for couplingto respective conventional input and output transmission lines 8 and 9,which latter lines may be coaxial transmission lines. At the end of thesubstrate sheet 3 opposite connectors 6 and 7, the strip conductors land 2 are coupled to impedance matched terminating resistors or energyabsorbers 4 and 5 for minimizing reflections. Signal generator 10 isadapted to supply excitation signals to transmission line 1 via resistor11 and coaxial line 8. The signal processing receiver 12 is coupled totransmission line conductor 2 via coaxial line 9 and supplies usefuloutput signals to a utilization device coupled to output terminal 13.Such output signals are generated 7 when an object contacts conductors 1and 2 or othertype of alarm, latchable or otherwise, or may be apedestrian counter or other suitable vehicular traffic control displayor device. As in FIG. 2 a counter circuit may be used to count the totalnumber of objects passing or the number per units of time, as desired.When switch 21 is properly set, the resultant count may be displayed bya conventional numerical display 22. On the other hand. the count may besupplied by switch 2I for use in a conventional vehicular trafficcontroller 23. As in FIG. 3, terminal 13 may be connected through switch21a to an alarm device 25 for sounding an alarm on the passage of asingle object or a predetermined number of objects. With switch 21aappropriately positioned, control device 26 may be operated for thepurpose of causing an actuator to open a door or to operate other servocontrolled devices.

Signal generator 10 may be selected from a variety of high frequencyoscillators well known in the art such, for example, as an IMPATT orimpact avalanche transit time diode oscillator operating in the L or Sband frequency spectrum. Other types of relatively stable, medium poweroscillators will be found suitable.

The signal processing receiver 12 consists of three 7 parts in seriesrelation, the first comprising a diode detector circuit 30, the second afilter circuit 31, and the third a threshold detector 32. Diode detectorcircuit 30 employs a rectifying diode with a resistor 41 connected tothe output of diode 40 to ground in shunt with a capacitor 42. Diodedetector circuit 30 normally produces a substantially constantunidirectional voltage for supply to filter 31. Filter 31 is a simplehigh-pass filter consisting of a series capacitor 43 whose output sideis coupled through resistor 44 to ground. Filter 31 responds only torelatively fast changes in the amplitude of the output of detectorcircuit 30, acting as a differentiator circuit to pass an impulse tothreshold circuit 32 when an object passes over transmission lineconductors 1, 2.

Threshold circuit 32 features a diode 47, which diode 47 is coupled toground and through series resistors 45 i and 46 to a suiable source ofbias voltage (not shown) connected to terminal 50. Diode 47 ispreferably a tunnel diode or other high speed diode adapted to serve asan impulse detector. A suitable diode has a negative resistancecurrent-voltage characteristic such that, under proper bias, the dioderesponse to the arrival of impulse emission from filter 31 is to moveabruptly into its region of instability, causing it to become highlyconductive.

In this manner, a current impulse of somewhat greater amplitude than theinput pulse, but of considerably longer duration is generated by tunneldiode 47 and is coupled to the input of one shot multivibrator circuit48; the longer duration, higher energy signal is desired for reliabletriggering of one shot multivibrator 48. The output pulse ofmultivibrator 48 is a rectangular pulse of, for example, 100 nanosecondduration which is passed to output terminal 13. The 100 nanosecond pulseis coupled also by lead 49 to the junction 51 between bias controlresistors 45 and 46. At junction 52, the trailing edge of the 100nanosecond pulse has the effect of resetting diode 47 and of stoppingconduction therethrough. Thus, tunnel diode 47 is reset to its originallow conduction state and is prepared to receive the next arrivingimpulse from filter 31 which exceeds the triggering level of diode 47.Other types of receiver elements performing the function of circuit 32may be employed such as, for example, receivers of the general typedescribed by KW. Robbins in the U.S. Pat. application Ser. No. 123,720for a Short Base- Band Pulse Receiver, filed Mar. 12, 1971, issued May9, 1972, as U.S. Pat. No. 3,662,316, and assigned to the Sperry RandCorporation.

As noted above, the presence sensor may consist of two spaced parallelstrip conductors I and 2 placed across the path of the object to bedetected. Detection of the object (a wheel of a vehicle or a shoe on ahuman foot, for example) occurs when the presence of the object causes achange or distortion in the electromagnetic-field-coupling patternbetween conductors I and .2. The sensor elements may, for instance, beplaced in a shallow groove in the surface of a passageway, road, or walkor it may be fastened directly to such a surface by a non-conductingcement such as an epoxy or other similar cement. A similar material maybe used to form a protective coating over conductors 1 and 2.

FIG. 4 and FIGS. 5 to 7 illustrate simple arrangements according to theinvention whereby the sensor may be permanently located in a passageway,roadway, or walkway. In FIG. 4, only a view of the output transmissionline 2 of the coupler device is seen, it being connected by an extension60 of line 2 to connector 7 and thus to coaxial line 9. Opposite theextension 60, line 2 is provided with a second extension 61 leading totermination 5 which may be buried in the body of the dielectricsubstrate layer 3. It will be understood that the input transmissionline 1 of the coupler may have similarly disposed input and terminatingtransmission line elements. The sensor structure may be permanentlyinlaid in a suitable depression in a roadway 58 made of concrete orother nonmetallic material.

As shown in FIG. 5, the invention may employ strip transmission linesimilar to commercial strip line, which is readily available in rigidform and may be laid in a shallow groove across the roadway orpassagewayto be monitored. Such conventional strip lines have goodthermal stability and relatively little susceptibility to environmentaleffects. Evidently, strip transmission lines of superior quality may beconstructed by a suitable choice of preferred conductive and dielectricmaterials, of which a wide variety is readily available.

In the system of FIG. 5, the dielectric slab 3 may constitute a solidrelatively non-flexible, low loss material such as a ceramic materialcoated on one side with a conductive ground plate 59 *madeof copper andsupporting on an upper surface the spaced copper strip lines 1 and 2.The upper surface 64 of dielectric slab 3 is arranged to be flush withthe upper surface of the roadway 58. It is seen that an object such as avehicle tire rolling over conductors l and 2 changes or distorts theelectromagnetic-field-coupling pattern between strip lines 1 and 2. Tothe extent that the pressure on the tire due to the weight of thevehicle causes the tire to flex into the gap 65 between conductors 1 and2, the degree of electromagnetic-field-coupling may be additionallyaltered over that when gap 65 is occupied only by air. In FIGS. 6 and 7,it is illustrated that the strip conductors l and 2 may be partially ortotally inlaid within the surface of dielectric slab 3, leaving an airgap 65 of lesser but finite depth in the instance of FIG. 7, but no airgap in the ease of FIG. 6. In certain applications, other clearlyrelated configurations may be found serviceable. In some instances, itmay be desirable to protect the strip transmission line sensor fromwater or dirt or the like. In others, such as those in which the sensoris used to monitor the passage of people, it may be particularlydesirable to hide the transmission line conductors l and 2 from view, asby use of a thin opaque rug or other dielectric covering overlyingconductors 1, 2.

In a simple arrangement for use in temporary installations, the sensormay take the form shown in FIG. 8, where conductors 1 and 2 are thinflexible conducting foils affixed to a flexible low loss sheet 63 ofmaterial such as cloth that may be unrolled across the path to bemonitored and even fastened to the surface of the flooring or roadway 58material. A thin film 59 of conductive foil is fastened to the bottom ofthe sensor before it is placed on the surface of roadway 58. It will-berecognized that the dimensions and proportions shown in the variousfigures are exaggerated for convenience in making the drawings clear,and that the dimensions and proportions are therefore not necessarilythose that would be used in actual practice. For example, the verticaldimensions in the cross sections of FIGS. 4 and 5 are exaggerated, sinceconductors l, 2, and 59 may be made of relatively thin metal foil.

As noted above, the transmission line systems may be constructed of flatmetal, electrically conducting strips 1 and 2 placed parallel to eachother on a suitable dielectric substrate 3. The widths of the conductorstrips 1 and 2 are determined by the desired impedance of thetransmission lines. For example, if the connecting coaxial cables 8 t?QtFIG- 1 ar 911m lisqsliqqhyl strip lines 1 and 2 will be chosen. Thewidth of gap 65 in FIG. is selected, for example, according to thedesired degree of coupling between conductors] and 2 and primarily uponthe type of devices chosen for generator 10 and the signal processorreceiver 12. For example, gap (65) widths of from 1/32 to 11/16 incheshave been found useful in various circumstances.

Still referring to FIG. 1, it is a feature of the present invention thatthe length D of the active coupling region defined by transmission lines1 and 2 is determined in a novel manner, being made equal to A /Z, whereA, is the operating wave length of oscillator 10 and therefore of thetransmission lines 1, 2. FIG. 9 represents the band widthcharacteristics of a coupler such as that of FIG. 1, the two curvesrepresenting the signal amplitude at output port connector 7 for a verysmall coupling coefficient between lines 1 and 2 (k l) and for asubstantially unity coupling (k 0.99). For

In conventional applications of such couplers, a wide signal spectrum isto be coupled at input port connector 6 and a broad band response isdesired of the coupler; then, the coupler selected is that having a wideflat response region about the median operating frequency at normallizedabscissa 0.5. For use in the present invention, a maximum coupling isagain usually selected, but the operating frequency is chosen tocorrespond to M/Z, which places the operating condition at thenormalized abscissa value I in FIG. 9 where there is zero output fromthe output port at connector 7 of FIG. 1.

The selected region adjacent abscissa value 1 provides rapid change inthe output amplitude, even in less-thanunity coupling couplers, and istherefore desirable for use in the present invention.

It is thus seen that the length D of the coupling region is determinedby the width of the passage or roadway to be monitored and that Ddetermines the operating frequency for oscillator 10. According to theinvention,

the value ).,,/2 is chosen so that when a tire, for in stance, passesover transmission lines 1, 2, the location of the null at abscissa I isdrastically disturbed, with output port 7 signal amplitude changing inthe order of or dB. The differentiator circuit 31 is sensitive to therate of change about the null characteristic, and not to the nullitself. Large signal changes are therefore supplied to thresholddetector 32, and the system is highly responsive. Furthermore, shouldoscillator 10 drift off the null frequency, significant changes canstill be observed from the output of differentiator 31 and the thresholddetector circuit 32, as a closed feed back loop, may be used to maintainthe null.

In the example of FIG. 1, it will beseen that for the average vehiclelane, a coupling region with a length D of about 10 feet mayreasonablybe employed such a value corresponding to a X12 wavelength formegacycles, a frequency value that is readily provided by an inexpensivediode or transistor oscillator. It will also be understood that thefunction illustrated in FIG. 9 is an ever repeating function, so thatnulls appear at normaliz'ed abscissa values l,2,3,4, ,n and so thatcorresponding choices of D could be made, such as It /2, A 3h /2, etc.However, the It /2 value would normally be used.

The novel presence sensor may be employed, for instance, in detectingvehicles or other objects moving along multi-lane passageways or roads,such as in Lane 1, Lane 2, or Lane 3 of the roadway 58 of FIG. 10. Inthe arrangement of FIG. 10, generator 10 excites a strip transmissionline 1 coupled in Lane 1 to the strip transmission line 2a, in Lane 2 tothe strip transmission line 2b, and in Lane 3 to the strip transmissionline 2c and provided with a termination 4. Thus, individual andindependent coupler regions are provided in each of the three lanes,using strip line 1 as a common excitation line. The output striptransmission lines 2a, 2b,and 2c are respectively provided with matchedterminations 5a, 5b, and 5c, and feed output signals to the individuallane signal processor receivers 12a, 12b, and 120 via planar or coaxiallines in the same general manner as Lane 2 produces a disturbance in thecoupling between strip lines 1 and 2b and generates an output which ismanipulated by signal processor receiver 12b to provide a transientoutput, as in FIG. 1, on output terminal 13b. The corresponding elementsfor Lanes 1 and 3 operate in a similar manner, producing a useful outputsignal on one of the respective terminals 13a and 13c in the presence ofa vehicle in the associated one of the respective Lanes 1 or 3.

FIG. 11 illustrates a triple lane arrangement somewhat similar to thatof FIG. 10, but wherein excitation power is fed in parallel to thesensors, rather than in series. The character of the drawing is also somodified as to be less schematic than that of FIG. 10 and to present aplan view showing the physical appearance of the several sensors. Signalgenerator 10 supplies energy to a conventional power divider which, inturn, feeds continuous wave energy through buried coaxial transmissionlines 8a, 8b, 80, to the respective coupler input transmission lines la,lb, and 10, each mounted on a corresponding dielectric substrate 3a, 3b,or 3c. In the presence of a vehicle, each such coupler is capable ofsupplying via an output line 2a, 2b, or 20 a transient output signal toan associated signal processor receiver 12a, 12b, or 12c over arespective buried coaxial transmission line 9a, 9b, or 9c. In thepresence of an input to one of the signal processor receivers 12a, 12b,12c, an output consequently appears at a corresponding terminal 13a,13b, 13c for application in utilization devices such as those of FIGS. 2and 3.

It will be seen that the three sensors of FIG. 11 do not necessarilyhave to be placed in collinear relation, but may be staggered onerelative to the others, depending upon the application involved. It willbe understood that the several terminations of the three sensors are notshown in FIG. 1 l, as it is assumed that they may be encapsulated orplaced below the surface of dielectric substrates 3a, 3b, and 3c. Itwill also be appreciated by those skilled in the art that the feed lines8a, 8b, and 8c of FIG. 11 and the receiver lines 9a, 9b, and 9c may beother than coaxial transmission lines; it is to be understood that thestrip transmission lines with their conductors flush with the surface ofthe roadway may also be used, all lines where coupling is not desiredbeing appropriately separated according to well known techniques.

It is seen that the invention is a versatile presence detection devicerelatively immune to interference by environmental electrical noisesignals, capable of recognition of the presence of a wide variety ofobjects including pedestrians and vehicles. The device involves sensingof presence by the measurement of a rapidly varying transmission linecoupling parameter, rather than sensing of slowly varying bridge orresonant circuit parameters. The novel sensor system of the presentinvention, while having good sensitivity, inherently has low false alarmcharacteristics and is suitable for application in a wide variety ofapplications.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than of limitation and that changes within thepurview of the appended claims may be made without departing from thetrue scope and spirit of the invention in its broader aspects.

We claim:

1. In combination:

a passageway surface means having a principal direction of passage,

dielectric substrate means having surface means substantially coplanarwith said passageway surface means,

signal generator means for supplying continuous wave signals,

transmission line energy coupling means excited by said continuous wavesignals comprising first and second transmission line conductor means offinite length spaced apart in substantially parallel energy couplingrelation on said substrate surface and lying substantially transverse ofsaid principal direction, said coupling means having a predeterminedcoupling characteristic alterable according to the proximity of anobject-overlying at least a portion of said coupling means, receivermeans for receiving presence detection signals in response to changes insaid coupling characteristic,

threshold detector means responsive to said receiver means for providingan output signal for signals generated by said receiver means ofamplitude greater than a predetermined amplitude, and

utilization means responsive to said thresholddetector means.

2. Transmission line energy coupling object presence detector meanshaving a predetermined high-frequency energy coupling characteristiccomprising:

dielectric substrate means having surface means adapted to be placed insubstantially coplanar relation with passageway surface means,

first and second substantially parallel energy coupling planartransmission line conductor means at said dielectric surface means fordefining a coupling region, said coupling region having a length D equalto A /Z, where )t is the preferred operating high frequency wave lengthfor said coupling means, said substrate means and said conductor meansbeing so constructed and arranged as to respond to said object whenoverlying at least a portion of said coupling means by a'change in thedegree of energy coupling between said conductor means, and meansresponsive to said change.

3. Apparatus as described in claim 2 wherein said dielectric substratemeans is placed within a recess in said passageway surface means.

4. Apparatus as described in claim 2 wherein said substrate meanssurface means includes a recessed portion between said conductor means.

1. In combination: a passageway surface means having a principal direction of passage, dielectric substrate means having surface means substantially coplanar with said passageway surface means, signal generator means for supplying continuous wave signals, transmission line energy coupling means excited by said continuous wave signals comprising first and second transmission line conductor means of finite length spaced apart in substantially parallel energy coupling relation on said substrate surface and lying substantially transverse of said principal direction, said coupling means having a predetermined coupling characteristic alterable according to the proximity of an object overlying at least a portion of said coupling means, receiver means for receiving presence detection signals in response to changes in said coupling characteristic, threshold detector means responsive to said receiver means for providing an output signal for signals generated by said receiver means of amplitude greater than a predetermined amplitude, and utilization means responsive to said threshold detector means.
 2. Transmission line energy coupling object presence detector means having a predetermined high-frequency energy coupling characteristic comprising: dielectric substrate means having surface means adapted to be placed in substantially coplanar relation with passageway surface means, first and second substantially parallel energy coupling planar transmission line conductor means at said dielectric surface means for defining a coupling region, said coupling region having a length D equal to lambda o/2, where lambda o is the preferred operating high frequency wave length for said coupling means, said substrate means and said conductor means being so constructed and arranged as to respond to said object when overlying at least a portion of said coupling means by a change in the degree of energy coupling between said conductor means, and means responsive to said change.
 3. Apparatus as described in claim 2 wherein said dielectric substrate means is placed within a recess in said passageway surface means.
 4. Apparatus as described in claim 2 wherein said substrate means surface means includes a recessed portion between said conductor means. 